23. Dependencies

from fastapi import FastAPI, Depends, Header, HTTPException

app = FastAPI()

# Simple dependency
def common_params(q: str = None, skip: int = 0, limit: int = 100):
    return {"q": q, "skip": skip, "limit": limit}

@app.get("/items/")
def read_items(commons: dict = Depends(common_params)):
    return commons

# Dependency with validation
def verify_token(x_token: str = Header(...)):
    if x_token != "secret":
        raise HTTPException(status_code=401)
    return x_token

@app.get("/protected/")
def protected(token: str = Depends(verify_token)):
    return {"message": "Access granted"}

# Class-based dependency
class CommonQueryParams:
    def __init__(self, q: str = None, skip: int = 0, limit: int = 100):
        self.q = q
        self.skip = skip
        self.limit = limit

@app.get("/users/")
def read_users(commons: CommonQueryParams = Depends()):
    return commons

# Sub-dependencies (Dependency graph)
def query_extractor(q: str = None):
    return q

def query_or_body_extractor(q: str = Depends(query_extractor)):
    return q or "default"

@app.get("/search/")
def search(query: str = Depends(query_or_body_extractor)):
    return {"query": query}